Whisk the flour, sugar, and salt. Cut in the cold butter until the mixture looks like coarse meal with some pea sized pieces. Stir in ice water gradually until the dough just holds together.

Cook the apples, brown sugar, cornstarch, cinnamon, nutmeg, and lemon juice in a saucepan over medium heat for 5 to 6 minutes until thickened and glossy. Off the heat, stir in the 1 tablespoon cold butter. Cool the filling completely.

On a floured surface, roll the chilled dough to ⅛ inch thick. Cut into 20 to 24 rounds of 3 to 3½ inches. Place half on a parchment lined baking sheet.

Spoon 1 tablespoon cooled filling onto each round on the sheet, leaving a border. Top with a second round, press the edges, crimp with a fork, and cut two small slits in the top. Brush with beaten egg.

Preheat oven to 375°F (190°C). Bake the cookies for 18 to 20 minutes until deep golden and bubbling. Cool on the sheet for 5 minutes, then transfer to a wire rack and serve warm.